An experimental animal model in which the course of immunodeficiency virus infection parallels the pathogenesis of the human disease is critical for the study of human AIDS. Simian immunodeficiency virus (SIV) induces an immunodeficiency syndrome in infected macaques that is remarkably similar in pathogenesis to human AIDS. An important use of this animal model system is the detailed study of pathogenesis and viral determinants of disease since many studies of this type are not feasible in humans. The purpose of this project is to investigate host and viral factors involved in variable disease progression in SIV-infected macaques and the lack of disease in African primates infected with their own strains of SIV. PATHOGENESIS OF SIVsm-INFECTION OF MACAQUES&#8232;To investigate the role of host factors in SIV-infection of macaques, we used a well-defined molecularly cloned virus (SIVsmE543-3), two macaque passages from the original sooty mangabey host. Previous studies in our lab have demonstrated that PBMC of individual macaques show vastly different susceptibility to SIVsmE543-3 infection in vitro and susceptibility is predictive of subsequent plasma viremia following inoculation. Our recent studies have now identified that allelic polymorphisms in the SPRY domain of TRIM5 alpha gene are responsible for much of this variation. In contrast, SIVmac which has been passaged in macaques multiple times appears to be adapted for resistance to multiple alleles of rhesus macaque TRIM5. We evaluated the effect of restrictive TRIM5 alleles (TRIM-TFP and TRIM-CypA) on viremia in a cohort of 43 rhesus SIVsmE543-3-infected rhesus macaques. The restrictive genotypes were associated with significantly lower viremia than observed in macaques with the permissive genotype, as well as emergence of escape mutations in the SIV capsid protein. The specific amino acid substitutions found in viruses that escaped from TRIM5 restriction were introduced into an E543-3 background in an effort to generate more reproducible viremia. These substitutions confer resistance to TRIM5 restriction by previously restrictive alleles in a single cycle assay and greater replication ability in primary PBMC of macaques homozygous for the resistant allele. We are currently evaluating the in vivo replication of this variant in TRIM-restrictive macaques. Preliminary studies show significantly higher peak plasma viremia This difference in susceptibility to TRIM5 polymorphism between SIVsm and SIVmac may explain the greater degree of protection achieved in immunized macaques challenged with repeated low dose intrarectal challenges with SIVsmE660 versus SIVmac251.&#8232;&#8232; Full length infectious clones of the related challenge stock, SIVsmE660 have also been generated by close to full length RT-PCR from the virus stock. Three of these viruses replicate efficiently in rhesus PBMC in vitro and very sensitive to neutralizing antibody (equivalent to Tier 1 classification used for HIV-1. All three are sensitive to the TRIM-TFP allele but two have mutations in the cyclophylin A binding loop that confer resistance to TRIM-CypA. Two clones were evaluated for in vivo viral replication and pathogenesis in TRIM-TFP/Q expressing rhesus macaques following intravenous inoculation. Both clones resulted in robust and persistent viremia that was associated with significant loss of CD4+ memory T cells. Evolution of the V1 and V4 regions was associated with escape from neutralization. Later stage variants had become more resistant to neutralizing antibody and might provide a virus that could be used in subsequent vaccine studies. A variant virus expressing this envelope has been generated and will be evaluated in macaques as a potential challenge strain. In addition, mutations associated with escape from TRIM-TFP have been introduced into one of the original clones and this will be evaluated in macaques in vivo. SIV NEUROPATHOGENESIS. SIV and HIV are both associated with the development of encephalitis. For HIV, the onset of AIDS dementia is generally a late stage finding. In contrast, most models of SIV encephalitis (SIVE) use animals that progress rapidly to disease and for that reason use pigtail macaques. We observed a small number of rhesus that developed SIVE over a more protracted course. The evolution of SIV in the cerebral spinal fluid (CSF) was compared with the virus that evolved in the plasma of two rhesus macaques that developed SIVE. While the virus in the CSF and plasma were similar during primary infection, distinct substitutions were observed sequentially in the two compartments and terminal virus from the brain and plasma were phylogenetically distinct. These findings are consistent with compartmentalization between the brain and blood during development of neuro-AIDS and the evolution of viruses with distinct genotypes and potentially distinct biological phenotypes in the brain. One of these isolates, SIVsmH631Br induced SIVE in one of four macaques; virus isolated from the brain off this animal (SIVsmH783Br) induced SIVE or meningitis in 4 of the 6 animals. Evolution of virus in the brain, CSF, blood and lymphoid tissues was evaluated using single genome amplification of selected samples and demonstrated compartmentalization of virus in the brain and CSF as compared to blood and plasma. An additional passage of virus (SIVsmH804E) has been inoculated into four rhesus and preliminary results show high plasma viremia and moderate CSF viremia. Studies are underway to study the molecular features of virus in the parenchyma and meninges using laser capture microdissection (LCM). Full-length infectious clones representative of this virus will be generated. &#8232; ASYMPTOMATIC INFECTION OF NATURAL HOST SPECIES. A second goal of this project is to study the mechanisms underlying the lack of pathogenicity of SIV for their natural host species, with emphasis on SIVagm from vervet monkeys. SIVagm is capable of inducing AIDS in PT macaques but African green monkeys (AGM) do not develop overt signs of disease following infection. We previously perturbed the development of adaptive immune responses by combined administration of anti-CD8 and CD20 antibodies during primary infection of PTM, and AGM and compared these animals to historical controls infected with the same virus. These data suggest that adaptive immune responses are critical in pathogenic SIV infection in PTM. However, the maintenance of a disease-free course of SIV infection in AGM likely depends on a number of mechanisms including non-adaptive immune mechanisms. &#8232;&#8232; In a collaboration with Jason Brenchley, we found that many of the CD4+ T cells from African green monkeys down-regulate CD4 in vivo as they enter the memory pool and that this occurs independent of SIV infection. These CD4-negative memory T cells maintain functions that are normally attributed to CD4 T cells including production of IL-2, production of IL-17, expression of FoxP3 and expression of CD40L; however lack of CD4 expression apparently protects these activated cells from infection by SIVagm in vivo. Thus the absence of SIV-induced disease progression in natural hosts species may be partially explained by preservation of a subset of T cells that maintain CD4 T cell function while being resistant to SIV-infection in vivo. We also observed that infant AGM have very few memory CD4+ T cells which might make them more resistant to SIV infection. Therefore we have been conducting small-scale breeding of SIVagm-infected AGM using pair housing; this has been successful with the production of 14 infants over a five year period. None of these infants became infected despite being nursed by their infected mothers for a year following birth. We have therefore initiated studies of SIVagm infection in infant AGM.